Effect of the molar ratio of (Ni2+ and Fe3+) on the magnetic, optical and antibacterial properties of ternary metal oxide CdO–NiO–Fe2O3 nanocomposites

In this work, the effect of the molar ratio of (Ni2+ and Fe3+) on the properties of CdO–NiO–Fe2O3 nanocomposites was investigated. The synthesis of CdO–NiO–Fe2O3 nanocomposites was carried out by self-combustion. XRD, UV–Vis, PL and VSM were used to describe the physical properties of the materials. The results showed significant progress in structural and optical properties supporting antibacterial activity. For all samples, the particle size decreased from 28.96 to 24.95 nm with increasing Ni2+ content and decreasing Fe3+ content, as shown by the XRD pattern, which also shows the crystal structure of cubic CdO, cubic NiO, and cubic γ-Fe2O3 spinel. The Ni2+ and Fe3+ contents in the CdO–NiO–Fe2O3 nanocomposites have also been shown to enhance the ferromagnetic properties. Due to the significant coupling between Fe2O3 and NiO, the coercivity Hc values of the samples increase from 66.4 to 266 Oe. The potential of the nanocomposites for antibacterial activity was investigated against Gram-positive (Staphylococcus aureus) and Gram-negative (Pseudomonas aeruginosa, Escherichia coli, and Moraxella catarrhalis) bacteria. Comparison of P. aeruginosa with E. coli, S. aureus and M. catarrhalis showed that it has a stronger antibacterial activity with a ZOI of 25 mm.

www.nature.com/scientificreports/ NiO is one of the most important transition metal oxides with a wide range of properties when reacting with polar surface materials and is used in a variety of applications due to its excellent chemical and thermal stability, antibacterial activity, environmental friendliness and industrial use 25 .
The capabilities of the individual metal oxides have been greatly enhanced by combining them into innovative nanocomposites, opening up new possibilities for applications in photocatalysis, electro-and optoelectronics, and biology 26 .
The synthesis of CdO-NiO-ZnO nanocomposites for photocatalytic and antibacterial properties was discussed by Karthik et al. Together with tested foodborne pathogens, the nanocomposite showed strong antibacterial activity 27 . Karthik et al. have reported CdO-NiO nanocomposites. The composite showed significant antibacterial activity against foodborne pathogens 28 . Tushar et al. reported the antibacterial activity of α-Fe 2 O 3 -ZnO in the core shell 29 . Balamurugan et al. reported the preparation of CdO-Al 2 O 3 -NiO nanocomposites for photocatalytic and magnetic properties. The composite exhibited weak ferrimagnetic assemblies, making it suitable for magnetic applications 30 . Gnanamoorthy et al. have reported rGO/ZnCo 2 O 4 nanocomposites and x-CuTiAP nanospheres for antimicrobial applications. The nanocomposites showed antimicrobial activity 31,32 .
This work aims to investigate the effects of the conditions for the preparation of CdO-NiO-Fe 2 O 3 nanocomposites by the self-combustion method on the structural, optical, magnetic and antibacterial activity.

Experiment
Materials. Cadmium  Characterizations. X-ray diffraction (XRD) was used to investigate the structural features of the fabricated samples (XD-2 X-ray diffractometer with Cu Kα (λ = 1.54 at 36 kV and 20 mA, China). A UV-Vis spectrophotometer (SPECORD 200) was used to measure the absorption spectra of the samples in the range of 190-1100 nm at room temperature. A spectrofluorometer (RF-5301PC; Shimadzu) with an excitation wavelength of 325 nm, an excitation and emission gap of 5 nm, an average scanning speed, and high sensitivity was used to record the photoluminescence spectra (PL) of the fabricated samples. The physical property measurement system (PPMS), QUANTUM DESIGN (MODEL6000), was used together with the attached vibrating magnetometer (VSM) to obtain the magnetic hysteresis (M-H) loops. The solvent for the prepared samples used to measure the absorption and photoluminescence spectra was dilute sulfuric acid (H 2 SO 4 ).

Antibacterial activity. The modified Kirby-Bauer disk diffusion test of the European Committee for
Antimicrobial Susceptibility Testing was used to investigate the antibacterial activities of CdO-NiO-Fe 2 O 3 nanocomposites against Gram-positive (Staphylococcus aureus) and Gram-negative (Pseudomonas aeruginosa, Escherichia coli, and Moraxella catarrhalis) bacteria 34 . Biochemical assays were used to further verify the identity of the isolates before testing the nanocomposites. The nanocomposites were serially diluted twice from the 75 mg/ml stock solution and suspended in sterile distilled water. The disks were impregnated with four different working dilutions. 450, 225, 112.5, and 56.25 μg/disk were prepared by impregnating a sterile filter paper disk (6 mm diameter) with 12 μl (6 μl on each side) for dilution. The plates were inoculated with swabs to form a uniform bacterial lawn on the agar surface. Using sterile forceps, the plates were positioned on the infected agar surface and incubated for 18-20 h at 37 °C. After completion of the incubation period, the diameters of the inhibition zones were measured to the nearest millimeter. In addition to the disks containing azithromycin as a positive control, a blank disk consisting solely of distilled water was used as a negative control. The XRD data obtained indicate that an increase in Ni 2+ and a decrease in Fe 3+ content causes a change in crystal lattice parameters and a decrease in crystallite size, as indicated in Table 2.
The Scherrer equation [38][39][40][41][42][43] is used to calculate the average crystal size of nanocomposites in the crystal plane of CdO (111), which can be given as follows: where K is the dimensionless form factor (K = 0.9), λ is the X-ray wavelength (= 0.1540 nm), β is the full width at half maximum (FWHM), and θ is the Bragg diffraction angle.
The microstrain (ε) of a nanocrystal is caused by defects in the nanocrystal, such as distortions and imperfections. The microstrain can be calculated using the following equation (ε) 43-45 : the dislocation density can be described by the following equation (δ) 39,43,45,46 : As seen in Table 2, the particle size decreased from 28.96 to 24.95 nm with increasing Ni 2+ and decreasing Fe 3+ content. The decrease in particle size of nanocomposites is attributed to the difference between the ionic radii of  www.nature.com/scientificreports/ Ni (0.074 nm), Cd (0.097 nm) and Fe (0.055 nm) 47 . The dependence of particle size on dislocation density and microstrain. The values of microstrain and dislocation density increase due to the large effect of particle size on the comprehensive stress of the nanocomposite 48 .
Optical properties. Absorption spectra. The absorption spectra of CdO-NiO-Fe 2 O 3 nanocomposites at different molar ratios of Ni 2+ and Fe 3+ were studied in the wavelength range of (200-800 nm) as shown in Fig. 2. The absorption peaks observed at 213-260 nm are attributed to the absorption band of CdO, while the absorption peaks observed at 310-320 nm are attributed to the absorption band of NiO in the nanocomposite 46 . In special CNF1 samples, a tiny absorption band at 530 nm was observed for Fe 2 O 3 . This absorption band is caused by the absorption of Fe 2+ and Fe 3+ ions of iron oxide 49 .
Band gap energy (Eg). As seen from Table 3 and Fig. 3, the optical band gap ( E g ) of the samples is between NiO (3.6 eV) 50 , CdO (2.5 eV) 51,52 and Fe 2 O 3 (2 eV) 53 . For the samples, the optical band gap ( E g ) increased with increasing Ni 2+ content and with decreasing Fe 3+ content. The decrease in band gap is related to the grain size. As localized energy states emerge and approach the conduction band, the energy band gap decreases in nanocomposites with a high content of Cd +246,48 .
PL study. Figure 4 shows the PL spectra of CdO-NiO-Fe 2 O 3 nanocomposites at 325 nm and room temperature. The near band edge (NBE) emission of NiO nanoparticles in a nanocomposite matrix was responsible for the observed UV emission peak at 359 nm 54 . Radiative recombination is responsible for the NBE peak in NiO in the exciton-exciton collision process 55 . The trapped electrons migrating into the valence band at the Ni interstitial are thought to be responsible for the strong violet emission peaks at 408 and 423 nm 40      In reality, the metal nanoparticles bind to the proteins and DNA of the pathogens by interacting with vital components such as the phosphorus (P) and sulphur (S) groups of bacterial DNA. As a result, bacterial DNA replication is destroyed 64 . One possible mechanism for the antibacterial effect is the production of free radicals. Through the damaged surface, the Cd 2+ , Ni 2+ and Fe 3+ ions in the nanocomposites penetrate the cell walls of the pathogens. Reactive oxygen species (ROS) are formed when ions are released from the nanoparticles. Superoxide radicals, hydroxyl radicals, singlet oxygen, and hydrogen peroxide are just some of the ROS components that have significant bactericidal activity [65][66][67][68][69][70][71] . The ZOI in this study compared with other studies is shown in Table 5.

Conclusion
In conclusion, the preparation of CdO-NiO-Fe 2 O 3 was successful, and its physical and antibacterial properties were studied. The molar ratio of Ni 2+ and Fe 3+ can affect the average crystallite size (D av ), dislocation density (δ) and microstrain (ε). In particular, the results showed that the coupling of CdO with NiO and Fe 2 O 3 improved the magnetic properties of CdO. At room temperature, the ferromagnetism of the CdO-NiO-Fe 2 O 3 nanocomposites was enhanced, making them suitable for magnetic applications. According to the results, the grown nanocomposite showed high performance as antibacterial activity for various Gram-negative and positive bacteria, which could be a strong candidate for bacterial disinfection.  www.nature.com/scientificreports/  www.nature.com/scientificreports/